The invention relates to a spark plug with combined surface discharge and air discharge gaps having a central electrode, an insulator surrounding the central electrode, and a ground electrode surrounding the insulator together with a spark plug body, wherein the insulator at its end portion is at a distance from the central electrode and forms a discharge chamber into which the central electrode extends, and the ground electrode surrounds the insulator around its end with a projection extending into the discharge chamber.
Such a spark plug is known from German Patent No. 3 544 176 and corresponding to U.S. Pat. No. 4,795,937. In this known spark plug the central electrode extends slightly into the discharge chamber which the insulator forms in that it extends in the axial direction of the plug over the central electrode, the insulator, at any rate in the end area of the central electrode, retaining a gap relative to it, and also the ground electrode, at any rate in the end region of the insulator, retaining a gap relative to the latter.
With such an arrangement there arise extensive sparking distances extending over the entire length of the discharge chamber and, with an adequately fast voltage rise at the spark plug capacitance and independently of the compression pressure of the mixture to be ignited, much ignition energy is converted in the gas and thereby practically long life is achieved.
The known spark plug according to German Patent No. 3 544 176, however, still suffers from the drawback that, on the starting of an internal combustion engine in which such a plug is used, high voltage is required, such as e.g. 30 kV. As, furthermore, with the known plug the surface ignition spark always runs over the ceramic insulator, this being associated with corresponding wear of the insulator, the life of the known plug is not yet optimum.